Ethylbenzene is a valuable product that is used mainly for the manufacture of styrene monomer. Most of the ethylbenzene is produced by alkylation of benzene with ethylene. Two types of reaction systems are commonly used for this alkylation. One type operates at high temperature in the vapor phase. The other operates at moderate temperatures in a liquid- or mixed-phase regime. When a high-temperature vapor-phase reaction system is employed, the alkylation reaction produces some quantity of iso- and normal-propylbenzenes which are undesirable impurities in the ethylbenzene. Iso-propylbenzene is also produced from the reaction of benzene and propylene contaminant in the feed ethylene. These propylbenzenes, and any reaction products which are formed from them in the styrene plant, have vapor pressures close to the styrene monomer and are impurities which are difficult to remove. These propylbenzenes can be separated from the ethylbenzene by distillation or other means but the byproduct stream containing the separated propylbenzenes will necessarily contain relatively large amounts of valuable ethylbenzene and polyethylbenzene which cannot economically be discarded. The polyethylbenzenes must be transalkylated with benzene and the propylbenzenes are preferably destroyed by reaction.
Fortunately the vapor-phase, high-temperature reactors used for ethylbenzene production also have the capability to transalkylate polyethylbenzenes and to destroy propylbenzenes. Complete destruction of propylbenzenes in a single pass is difficult to achieve, but the per-pass destruction rate of propylbenzenes is high enough that the build-up of unreacted propylbenzenes is modest. A large buildup, resulting from a very low per-pass destruction rate, would necessitate significantly larger separation equipment and correspondingly larger operating costs for the separation. Therefore, processes that use high-temperature vapor-phase reactors for the production of ethylbenzene have recycled the separated propylbenzenes to the same reactor, or to a separate high-temperature vapor-phase reactor, in order to take advantage of the relatively good capability of such reactors to destroy propylbenzenes.
A disadvantage of the use of high-temperature vapor-phase reactors for the production of ethylbenzene is the production in the reactor of xylenes. Xylenes cannot be economically separated from the ethylbenzene product, and, furthermore, are difficult to separate from the styrene monomer that is produced from the ethylbenzene.
Processes that react benzene with ethylene in the liquid phase, or in a mixed-phase reactor, at temperatures of 150-280.degree. C., do not produce xylene impurities and are therefore used when an ethylbenzene product of superior quality is desired. These processes use similar conditions for the transalkylation of polyethylbenzenes with benzene. At the operating temperatures of such processes, high levels of propylbenzenes are not produced when the ethylene feedstock used does not contain significant levels of propylene. Their separation from the ethylbenzene product is therefore unnecessary. This is fortunate, since at the reaction conditions used in such processes, the destruction rates of propylbenzenes are low.
The corresponding disadvantage of liquid-phase and mixed-phase processes is therefore an inability to deal with feeds containing significant levels of propylbenzenes, or components that make propylbenzenes, such as propylene. As the end-uses of styrene monomer demand ever-increasing levels of purity, producers of ethylbenzene increasingly desire to make a product with the lowest practical content of xylenes, and are turning to liquid- or mixed-phase processes to achieve this. On the other hand, they also desire to use less expensive ethylene feedstocks, such as from fluid catalytic cracking units, which contain significant levels of propylene. Some producers, with existing vapor-phase alkylation units, are seeking to transalkylate the polyethylbenzenes produced in such units at liquid-phase conditions, in order to achieve a reduction in the xylenes content of their product. Hitherto, this has not been possible because of the aforementioned inability of liquid-phase processes to destroy propylbenzenes.